1. Field of the Invention
The present invention relates to a mobile communication system. More particularly, the present invention relates to a method and an apparatus by which a User Equipment (UE) can identify a service for a certain frequency layer.
2. Description of the Related Art
The recent rapid development of the mobile telecommunication system has enabled the rapid increase in the quantity of data provided through the mobile telecommunication system, which has thereby induced the development of the third generation (3G) mobile communication system for transmitting data at a higher speed or data rate. For the 3G mobile communication system, European countries have employed a Wideband Code Division Multiple Access (WCDMA) scheme, which is an asynchronous scheme between base stations, and North American countries have employed a Code Division Multiple Access (CDMA) scheme, which is a synchronous scheme between base stations, respectively, as a wireless access standard.
In such a mobile communication system, a plurality of users simultaneously share a wide frequency band. Specifically, a transmitter simultaneously transmits a signal converted over a wideband by using a spread spectrum scheme to users and a receiver detects a corresponding signal for each user by using a corresponding code or sequence. The CDMA scheme has better security than the other multiple access schemes have because the transmitted data is not easily divulged in the CDMA scheme.
The mobile communication system supports a broadcast/multicast service in which one data source provides the same data stream to a plurality of UEs. The broadcast/multicast service comprises a cell broadcast service for mainly providing messages and a Multimedia Broadcast/Multicast Service (MBMS) for providing real time images and voice, still image and text in the type of multimedia.
When multiple MBMSs are overlapping frequency bands within the same area, a UE must perform a service selection process in order to benefit from one of the MBMSs.
Hereinafter, reference will be made to FIG. 1 illustrating an example of a cell layer structure comprises cell frequencies overlapping in one service area.
In the cell structure illustrated in FIG. 1, different cell frequencies including frequency #1 (f1) 104, frequency #2 (f2) 103, frequency #3 (f3) 102 and frequency #4 (f4) 101 overlap in one service area. The UE 111 joins both service A and service B. Service A is a service that can be provided in all frequencies, and service B is a service that can be provided in only frequency #4 (f4) 101. The service B may be an MBMS and can be provided only in a particular frequency for efficient use of radio resources and due to the characteristics of the service.
If service B starts while the UE receives service A, the system transmits a message instructing a frequency change (which implies cell change) through frequency #4 (f4) 101 because service B can be provided in only frequency #4 101. The frequency change message preferably comprises a service identifier (ID), changeable frequency (or cell information), and the like. Then, the UE 111 in frequency #2 (f2) 103 performs a service selection because it cannot simultaneously receive both service A and service B. The service selection may be a service selection in a Non Access Stratum (NAS) level through interconnection with NAS or a selection based on priorities of services in Access Stratum (AS).
For example, when the UE performs service selection, if service A has a higher priority than service B, the UE does not move to frequency #4 (f4) 101. Then, the UE cannot simultaneously receive both service A nor service B in the current frequency #2 (f2) 103. That is, the UE can receive only service A and cannot receive service B. As a result of the service selection, if service A is selected, the UE stays in the current frequency #2 (f2) 103 and continuously receives service A while giving up service B.
According to the method as described above in which the UE maintains the current frequency through the unnecessary service selection, the UE must abandon some service (service B in the example) in spite of the availability of all joined services (service A and service B).